Propylene polymers are having increasing success as materials for the manufacture of articles for use in healthcare and medical applications. These articles include containers, such as bottles, bags and pouches, for biologic liquids, syringes and other items that can be sterilized by autoclaving. The material's success is due, in part, to a good balance of mechanical properties and resistance to sterilization treatment. It is important that the properties of the material used for this kind of application do not deteriorate after heat treatment for sterilization, and medical/healthcare containers should maintain their transparency after sterilization to enable inspection of their content.
Several attempts have been made to improve the sterilization resistance of propylene polymers, and improvements often result in a higher complexity of the base material.
EP 1849826 B1, for example, describes articles for sterilization comprising a polyolefin composition containing a heterophasic propylene copolymer and an ethylene homopolymer.
EP 2176340 B1 relates to sterilizable polypropylene compositions comprising a matrix resin and an elastomeric resin as the dispersing phase, wherein the matrix resin comprises a propylene homopolymer and a propylene copolymer.
WO 2012/084768 describes a propylene polymer composition having a good balance of elasticity and transparency for use in the preparation if infusion bottles. The composition comprises 60-90% of a crystalline copolymer of propylene, with 1.0-5.0% of ethylene-derived units and 10-40% of a copolymer of propylene with 18-32% of ethylene-derived units. The composition has a melt flow rate value of 1.0-2.0 g/10 min.
It has now been found that, by using specific propylene random copolymers, manufactured articles can be obtained that exhibit surprising optical properties (transparency) after heat sterilization based, in part, on the unexpected finding that the transparency as a function of the comonomer amount shows a deviation from linear behavior.